1. Field of Endeavor
The present disclosure relates to a regenerative medium voltage inverter.
2. Background
This section provides background information related to the present disclosure which is not necessarily prior art.
In general, equipment referred to as a power converter, inverter or drive is employed to provide power to another piece of equipment such as a motor. Specifically, such an inverter (inverter is used generally herein to refer to inverters, converters, drives) is coupled to a utility connection to receive incoming input power such as a three-phase AC (Alternating Current) power. Furthermore, a medium voltage inverter is an inverter having an input power whose rms (root mean square) value is over 600V for a line-to-line voltage, and is generally used to drive an industrial load of large inertia of, for a non-limiting, fans, pumps and compressors.
In these application fields, variable speed operations frequently occur, where regenerative operations are generated, if a fast acceleration or a fast deceleration is required.
FIG. 1 is a configurative view of a series H-bridge medium voltage inverter according to prior art, where the inverter is configured with a three-step unit power cells.
A conventional medium voltage inverter (100) receives a 3-phase power from an input power unit (200) and supplies the power to a motor (300). The input power unit (200) provides a 3-phase whose rms (root mean square) value is over 600V for a line-to-line voltage. The motor (300) is a 3-phase high voltage motor, and may be an induction machine or a synchronous machine.
A phase switching transformer (110) provides a galvanic isolation between the input power unit (200) and the medium voltage inverter (100), reduces harmonics at an input terminal and provides an input 3-phase power adequate to each unit power cell (120). The unit power cell (120) receives the power from the phase switching transformer (110) and outputs a phase voltage to the motor (300), where each unit power cell (120) is composed of a group.
Referring to FIG. 1, the power cells A1, A2 and A3 are connected in series at an output voltage to synthesize ‘a’ phase voltage of the motor (300), the power cells B1, B2 and B3 are connected in series at an output voltage to synthesize ‘b’ phase voltage of the motor (300), and the power cells C1, C2 and C3 are connected in series at an output voltage to synthesize ‘c’ phase voltage of the motor (300). The synthesized ‘b’ phase voltage and ‘a’ phase voltage are apart with a 120 degree phase difference, and the synthesized ‘c’ phase voltage and ‘b’ phase voltage are also apart with a 120 degree phase difference.
FIG. 2 is a configurative view illustrating a unit power cell of FIG. 1. Referring to FIG. 2, the unit power cell (120) includes a 3-phase diode rectifier (121), a DC-link capacitor (122) and an inverter (123). The 3-phase diode rectifier (121) outputs a 3-phase rectified DC voltage using an output voltage of the phase switching transformer (110) as an input. The DC-link capacitor (122) stores the input power of the 3-phase diode rectifier (121). The inverter (123) is a single phase full bridge inverter to synthesize output voltages via switching of the switching elements (123a˜123d) using the voltages received from the DC-link capacitor (122).
Now, system of FIGS. 1 and 2 will be described.
The phase switching transformer (110) converts a phase and size of a high voltage input power catering to requirement of the unit power cell (120). An output voltage of the phase switching transformer (110) becomes of an input power of each unit power cell (120) and is converted to a DC through the 3-phase diode rectifier (121) of FIG. 2. The DC-link capacitor (122) serves to constantly maintain the output voltage of the 3-phase diode rectifier (121).
The single phase full bridge inverter (123) synthesizes the AC output voltages with the voltages from the DC-link capacitor (122). IF the voltage of the DC-link capacitor (122) is assumed as ‘E’, an output voltage of the inverter (123) would be shown as ‘E’, ‘O’ and ‘−E’ according to switching state.
For a non-limiting example, if 123a and 123d are electrically conducted, the synthesized output voltage is ‘E’, and if 123b and 123c are electrically conducted, the synthesized output voltage is ‘−E’, if 123a and 123c, or 123b and 123d are electrically conducted, the synthesized output voltage is ‘0’.
In the unit power cell structure of FIG. 1, output voltages of A1, A2, A3, B1, B2, B3 and C1, C2, C3 are all serially connected, such that serially connected output phase voltages are synthesized in 7 steps of ‘3E’, ‘2E’, ‘E’, ‘0’, ‘−E’, ‘−2E, and ‘−3E’. An output line-to-line voltage of the motor from the synthesized output phase voltages may be synthesized in 13 steps of ‘6E’, ‘5E’, ‘4E’, ‘3E’, ‘2E’, ‘E’, ‘0’, ‘−E’, ‘−2E’, ‘−3E’, ‘−4E’, ‘−5E’, ‘−6E’, which may be generalized as below.m=2H+1  [Equation 1]p=2m−1=4H+1  [Equation 2]where, m is a level number of output phase voltage, H is the number of unit power cells (120) installed at each phase of motor (300), and p is the level number of output line-to-line voltages. Meanwhile, with an output of each unit power cell (120) being an output of a single full bridge inverter, an output of a DC-link power is formed with a voltage ripple. First, an output voltage and an output current of each cell are defined as under.vO=√{square root over (2)}VO sin ωt  [Equation 3]io=√{square root over (2)}Io sin(ωt−φ)  [Equation 4]where, φ is a load angle, ω is an operating frequency, t is time, VO and Io are an output voltage and an rms value of an output current. An output power of unit power cell (120) can be obtained as below based on the Equations 3 and 4.po=voio=VoIo cos φ−VoIo cos(2ωt−φ)  [Equation 5]
As noted from the Equation 5, it can be learned that an output of unit power cell (120) is divided to a DC component of VoIo cos φ and an AC component of VoIo cos(2ωt−φ), where the AC component has a ripple corresponding to twice that of the operating frequency. Now, a current flowing in DC link can be obtained as below.
                              i                      D            ⁢                                                  ⁢            C                          =                                            p              o                                      v                              D                ⁢                                                                  ⁢                C                                              =                                                                                          V                    o                                    ⁢                                      I                    o                                                                    v                                      D                    ⁢                                                                                  ⁢                    C                                                              ⁢              cos              ⁢                                                          ⁢              ϕ                        -                                                                                V                    o                                    ⁢                                      I                    o                                                                    v                                      D                    ⁢                                                                                  ⁢                    C                                                              ⁢                              cos                ⁡                                  (                                                            2                      ⁢                      ω                      ⁢                                                                                          ⁢                      t                                        -                    ϕ                                    )                                                                                        [                  Equation          ⁢                                          ⁢          6                ]            
Based on the Equations 5 and 6, it can be learned that the DC link is generated with a ripple twice that of the operating frequency.
The conventional series H-bridge medium voltage inverter thus described suffers from a disadvantage in that, because it is formed with a diode rectifier for an input terminal of a unit power cell, it is impossible to perform a regenerative operation to make it difficult to perform a fast acceleration or a fast deceleration.
As a result, the conventional series H-bridge medium voltage inverter thus described has a disadvantage of taking a long time for decelerated operation during a decelerated operation. Another disadvantage is that each capacitance of DC-link capacitor of all unit power cells must be enlarged to increase the size of an entire system.
Thus, there is a need to address the abovementioned disadvantages.